Stormwater wetland construction guidance

Stormwater Wetland Construction Guidance
Stormwater wetlands perform well in reducing peak flows and pollutant removal when properly designed and constructed. These wetland construction guidelines are based on experience gained at more than 30 sites across North Carolina. stormwater wetlands to provide efficient of providing maximum benefits and being and maximize environmental benefits.
Stormwater wetlands are thriving ecosystems.2 Urban Waterways
tems often vary considerably. Specifically, we will focus
• Planning
• Oversight
• Sequencing
• Practices — including site layout, inlet and outlet
CONSTRUCTION PLANNING
Proper planning will help to get your stormwater wet
the first growing season. Planting early in the growing
Outline the specifics of construction in a guidance
sequences. Providing this document to potential contrac
to discuss the details of the project, can be very benefi
condition will help influence the perspective of potential
contractors. Open and thorough communication on site
Figure 1. Meeting with contractors and stakeholders before
construction will help stormwater wetland projects begin
smoothly, saving time and money.
ESTIMATING THE NUMBER OF WETLAND PLANTS
The number of plants needed for a project can be easily esti-mated
based on the wetland’s surface area and the required
spacing that your budget will allow. Numerous plant spacing
calculators are available via the Internet. Most use a simple
calculation based on the following relationship:
Number of plants = Planting area (ft2) ÷ (Plant spacing (ft))2
You can use the following formulas derived from this relation-ship
to calculate the number of plants required:
12-inch spacing:
No. plants = sq ft of wetland area
No. plants = 43,560 × acres of wetland
24-inch spacing:
No. plants = 0.25 × sq ft of wetland area
No. plants = 10,890 × acres of wetland
36-inch spacing:
No. plants = 0.11 × sq ft of wetland area
No. plants = 4,841 × acres of wetland
Planting on 12-inch centers will ensure widespread vegetative
coverage within 1 year; 24- and 36-inch centers tend to be
adequate for coverage in 2 and 3 years, respectively.
At least 6 months before construction, notify any local nursery
or nurseries that will provide plants of the varieties and num-bers
you will need for your project to make sure the plants will
be available (particularly if your project is sizable).
Stormwater Wetland Construction Guidance 3
handled—will it be kept on-site or transported off-site?
CONSTRUCTION OVERSIGHT
stormwater wetland is constructed within specified
ence of a few inches can make a significant impact
calls for the wetland to partially infiltrate, or properly
compacted if low infiltration is desired. Additionally,
changes that are beneficial to the project’s timetable
CONSTRUCTION SEQUENCING
A plan that outlines the most efficient sequence for con
1. Site layout
All existing utilities at the site must be identified and
Figure 2. Get to know your contractors, and listen to their sug-gestions.
Doing so will be beneficial to your project.
Figure 3. Stake out the wetland perimeter, internal wetland fea-tures,
and locations of structures prior to excavation. Make sure
all existing utilities at the site are located and marked.
2. Erosion control measures
4 Urban Waterways
Figure 4. Installation of a silt fence along a stream bank near a
new stormwater wetland construction site
3. Outlet construction
(AG-588-12) that offer flex
wetland. If inflows can be diverted around the proposed
to be “in-line” with stormwater flow, outlet structures
4. Excavation and wetland soil preparation
or soil additions (such as bentonite) to reduce infiltra
5. Bank stabilization
Figure 5. Stabilize the banks each day following excavation of a
stormwater wetland.
6. Inlet stabilization
to dissipate the stormwater’s energy as it enters the
7. Final surface preparation
unavoidable construction traffic or crusted during
8. Planting
9. Water management
Stormwater Wetland Construction Guidance 5
Figure 8. Examples of outlet structures built on-site
(top) and prefabricated and delivered to the
site (bottom)
Figure 6. The soils for this stormwater wetland are properly
prepared for water and wetland plants.
Figure 7. Successful vegetation establishment greatly depends
on good topsoil condition, plant spacing, and proper water
management.
CONSTRUCTION PRACTICES
Installation of outlet structures
a flashboard riser with tongue-in-groove boards to allow
down orifice protected by a trash rack to reduce clog
compaction, and soils. Prefabricated outlet structures
should be installed on a firm base for stability. Depend
clay, or compacted gravel footing underlain with filter
water level control. For example, a flashboard riser-type
6 Urban Waterways
preferential flow of water over one side of the structure
dissipate energy from the stormwater wetland outflow.
Excavation and wetland soil preparation
techniques
and budget standpoints. Problems generally occur when
multiple times, or when the wetland’s internal features
Figure 10. Armoring downstream of the outlet structure will
reduce the potential for erosion that could threaten stability.
Figure 9. Installation of a large flashboard riser outlet
structure
Figure 11. Keep dump trucks moving for off-site
transport of soil (top), and keep the excavator opera-tor
updated on correct grade to speed construction
(bottom).
Stormwater Wetland Construction Guidance 7
grading progresses will limit traffic in the wetland, and
ing the operator quickly carve out the wetland’s internal
features and shape side-slopes. On projects where major
Often, however, designed soil cuts for the wetland
expose nutrient deficient subsoil. Therefore, during
addition. Once the subsurface elevations have been
finalized, the topsoil can then be added or replaced.
sites intended to infiltrate some stormwater, the operator
within the excavated wetland area. Plan excavation stra
tegically so that the equipment’s weight does not com
replacement on the first pass with the excavator on high
wetland later to finish these tasks.
Figure 12. If possible, keep the excavator out of the wetland
and use the bucket teeth for excavation to keep the wetland
soil loose for planting. Remember to stockpile topsoil for
replacement!
the bottom and banks of the pond. Operators
Exfiltration from BMPs,
wetland also must be excavated lower than final grade
stormwater inflow, so topsoil elevations 0.1 foot above
Site stabilization
topsoil will erode along the wetland’s banks, costing
Figure 13. Surveying a properly prepared wetland surface.
8 Urban Waterways
fibers with biodegradable mesh). This fabric is very
grass through the fibers, and will degrade quickly.
slope. Certain stormwater wetland retrofits require that
the catch basins, the water will flow within the plastic
slope drains instead of flowing down the steep banks,
significantly reducing erosion. As the banks become
Inlet structures
deep pool near the wetland entrance that slows inflow
beneficial to plant vegetation near the inlet, but outside
Figure 14. Stabilize banks daily with lightweight fab-ric
and seeding to reduce the chance of erosion while
the wetland develops.
Figure 15. Two examples of using slope drains for
bank stabilization. Surface water generated during
larger storms flowing into the wetland is directed
into the drains. The drainage reduces erosion that
would result from concentrated flow down the banks.
Stormwater Wetland Construction Guidance 9
stability. If the wetland also receives inflow from other
Planting
wetland zones. Planting on 24-inch centers is recom
if the wetland is planted on 12-inch centers. Planting
Planting of a stormwater wetland can be completed
with more efficiency and skill. Volunteers will be less
some plant species may arrive in flats, while others may
released just prior to planting. The wetland floor should
Figure 16. Use riprap of granite or marl to slow inflow to
the wetland and protect the inlet from erosion.
be identified before the volunteer or professional plant
ers arrive. Mark these areas with flagging or with mark
flats or bags into their target planting areas. This will not
Figure 17. Planting a stormwater wetland can be a community
volunteer effort.
10 Urban Waterways
ing water on hand. Orient the volunteers to the loca
Early wetland establishment tips
Once a stormwater wetland has been constructed and
it’s a good idea to budget for revegetation or repairs
during the project’s first year.
4 to 6 inches during the beginning of the first and sec
to flood the temporary inundation zones from a source
the wetland’s banks also receive water regularly, for
to a wetland’s stability. If heavy rainfall events occur
right after a project’s completion, even the best erosion
from surface water flowing into the wetland. If this hap
matting reapplied. One way to protect against erosion
specific. If you suspect or discover wildlife damage,
Mainte
nance of Stormwater Wetlands and Wet Ponds,
Figure 18. Keep water levels relatively low during
early establishment (top), and be prepared to irri-gate
through flooding or a sprinkler system during a
drought (bottom).
Stormwater Wetland Construction Guidance 11
wetland project, it is important to be firm in sticking
ideas that can also benefit project success.
RESOURCES
588-12). Online: http://www.bae.ncsu.edu/stormwa
ter/PublicationFiles/WetlandDesignUpdate2007.pdf
and Wet Pond Maintenance (AGW-588-7). Online:
http://www.bae.ncsu.edu/stormwater/Publication
(AG-588-2). Online:
http://www.bae.ncsu.edu/storm¬water/Publication
edu/stormwater N.C. State University’s clearinghouse
for stormwater BMP guidance, including design, con
Figure 19. Adding features and topsoil in a large
wetland. Side-slopes have already been prepped, and
grass is coming up (top). After 1 year, vegetation is
successfully established (bottom).
Mature plants beautify a stormwater wetland.
established in the first growing season, plant growth
wetland a permanent home. If you find that wildlife
CONCLUSIONS
Stormwater wetlands are important stormwater BMPs
with care. Proper construction planning saves time
and money. Practices discussed and recommended in
NC STATE UNIVERSITY
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Published by
NORTH CAROLINA COOPERATIVE EXTENSION
11-CALS-2218 AGW-588-13
11/10—BS/KEL
Prepared by
Michael R. Burchell II, Assistant Professor and Extension Specialist
William F. Hunt, P.E., Associate Professor and Extension Specialist
Kristopher L. Bass, P.E., Extension Associate
Department of Biological and Agricultural Engineering
North Carolina State University
Jason Wright, P.E., Water Resources Engineer,
TetraTech, San Diego, Calif.
250 copies of this public document were printed at a cost of $595 or $2.38 per copy.

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Stormwater Wetland Construction Guidance
Stormwater wetlands perform well in reducing peak flows and pollutant removal when properly designed and constructed. These wetland construction guidelines are based on experience gained at more than 30 sites across North Carolina. stormwater wetlands to provide efficient of providing maximum benefits and being and maximize environmental benefits.
Stormwater wetlands are thriving ecosystems.2 Urban Waterways
tems often vary considerably. Specifically, we will focus
• Planning
• Oversight
• Sequencing
• Practices — including site layout, inlet and outlet
CONSTRUCTION PLANNING
Proper planning will help to get your stormwater wet
the first growing season. Planting early in the growing
Outline the specifics of construction in a guidance
sequences. Providing this document to potential contrac
to discuss the details of the project, can be very benefi
condition will help influence the perspective of potential
contractors. Open and thorough communication on site
Figure 1. Meeting with contractors and stakeholders before
construction will help stormwater wetland projects begin
smoothly, saving time and money.
ESTIMATING THE NUMBER OF WETLAND PLANTS
The number of plants needed for a project can be easily esti-mated
based on the wetland’s surface area and the required
spacing that your budget will allow. Numerous plant spacing
calculators are available via the Internet. Most use a simple
calculation based on the following relationship:
Number of plants = Planting area (ft2) ÷ (Plant spacing (ft))2
You can use the following formulas derived from this relation-ship
to calculate the number of plants required:
12-inch spacing:
No. plants = sq ft of wetland area
No. plants = 43,560 × acres of wetland
24-inch spacing:
No. plants = 0.25 × sq ft of wetland area
No. plants = 10,890 × acres of wetland
36-inch spacing:
No. plants = 0.11 × sq ft of wetland area
No. plants = 4,841 × acres of wetland
Planting on 12-inch centers will ensure widespread vegetative
coverage within 1 year; 24- and 36-inch centers tend to be
adequate for coverage in 2 and 3 years, respectively.
At least 6 months before construction, notify any local nursery
or nurseries that will provide plants of the varieties and num-bers
you will need for your project to make sure the plants will
be available (particularly if your project is sizable).
Stormwater Wetland Construction Guidance 3
handled—will it be kept on-site or transported off-site?
CONSTRUCTION OVERSIGHT
stormwater wetland is constructed within specified
ence of a few inches can make a significant impact
calls for the wetland to partially infiltrate, or properly
compacted if low infiltration is desired. Additionally,
changes that are beneficial to the project’s timetable
CONSTRUCTION SEQUENCING
A plan that outlines the most efficient sequence for con
1. Site layout
All existing utilities at the site must be identified and
Figure 2. Get to know your contractors, and listen to their sug-gestions.
Doing so will be beneficial to your project.
Figure 3. Stake out the wetland perimeter, internal wetland fea-tures,
and locations of structures prior to excavation. Make sure
all existing utilities at the site are located and marked.
2. Erosion control measures
4 Urban Waterways
Figure 4. Installation of a silt fence along a stream bank near a
new stormwater wetland construction site
3. Outlet construction
(AG-588-12) that offer flex
wetland. If inflows can be diverted around the proposed
to be “in-line” with stormwater flow, outlet structures
4. Excavation and wetland soil preparation
or soil additions (such as bentonite) to reduce infiltra
5. Bank stabilization
Figure 5. Stabilize the banks each day following excavation of a
stormwater wetland.
6. Inlet stabilization
to dissipate the stormwater’s energy as it enters the
7. Final surface preparation
unavoidable construction traffic or crusted during
8. Planting
9. Water management
Stormwater Wetland Construction Guidance 5
Figure 8. Examples of outlet structures built on-site
(top) and prefabricated and delivered to the
site (bottom)
Figure 6. The soils for this stormwater wetland are properly
prepared for water and wetland plants.
Figure 7. Successful vegetation establishment greatly depends
on good topsoil condition, plant spacing, and proper water
management.
CONSTRUCTION PRACTICES
Installation of outlet structures
a flashboard riser with tongue-in-groove boards to allow
down orifice protected by a trash rack to reduce clog
compaction, and soils. Prefabricated outlet structures
should be installed on a firm base for stability. Depend
clay, or compacted gravel footing underlain with filter
water level control. For example, a flashboard riser-type
6 Urban Waterways
preferential flow of water over one side of the structure
dissipate energy from the stormwater wetland outflow.
Excavation and wetland soil preparation
techniques
and budget standpoints. Problems generally occur when
multiple times, or when the wetland’s internal features
Figure 10. Armoring downstream of the outlet structure will
reduce the potential for erosion that could threaten stability.
Figure 9. Installation of a large flashboard riser outlet
structure
Figure 11. Keep dump trucks moving for off-site
transport of soil (top), and keep the excavator opera-tor
updated on correct grade to speed construction
(bottom).
Stormwater Wetland Construction Guidance 7
grading progresses will limit traffic in the wetland, and
ing the operator quickly carve out the wetland’s internal
features and shape side-slopes. On projects where major
Often, however, designed soil cuts for the wetland
expose nutrient deficient subsoil. Therefore, during
addition. Once the subsurface elevations have been
finalized, the topsoil can then be added or replaced.
sites intended to infiltrate some stormwater, the operator
within the excavated wetland area. Plan excavation stra
tegically so that the equipment’s weight does not com
replacement on the first pass with the excavator on high
wetland later to finish these tasks.
Figure 12. If possible, keep the excavator out of the wetland
and use the bucket teeth for excavation to keep the wetland
soil loose for planting. Remember to stockpile topsoil for
replacement!
the bottom and banks of the pond. Operators
Exfiltration from BMPs,
wetland also must be excavated lower than final grade
stormwater inflow, so topsoil elevations 0.1 foot above
Site stabilization
topsoil will erode along the wetland’s banks, costing
Figure 13. Surveying a properly prepared wetland surface.
8 Urban Waterways
fibers with biodegradable mesh). This fabric is very
grass through the fibers, and will degrade quickly.
slope. Certain stormwater wetland retrofits require that
the catch basins, the water will flow within the plastic
slope drains instead of flowing down the steep banks,
significantly reducing erosion. As the banks become
Inlet structures
deep pool near the wetland entrance that slows inflow
beneficial to plant vegetation near the inlet, but outside
Figure 14. Stabilize banks daily with lightweight fab-ric
and seeding to reduce the chance of erosion while
the wetland develops.
Figure 15. Two examples of using slope drains for
bank stabilization. Surface water generated during
larger storms flowing into the wetland is directed
into the drains. The drainage reduces erosion that
would result from concentrated flow down the banks.
Stormwater Wetland Construction Guidance 9
stability. If the wetland also receives inflow from other
Planting
wetland zones. Planting on 24-inch centers is recom
if the wetland is planted on 12-inch centers. Planting
Planting of a stormwater wetland can be completed
with more efficiency and skill. Volunteers will be less
some plant species may arrive in flats, while others may
released just prior to planting. The wetland floor should
Figure 16. Use riprap of granite or marl to slow inflow to
the wetland and protect the inlet from erosion.
be identified before the volunteer or professional plant
ers arrive. Mark these areas with flagging or with mark
flats or bags into their target planting areas. This will not
Figure 17. Planting a stormwater wetland can be a community
volunteer effort.
10 Urban Waterways
ing water on hand. Orient the volunteers to the loca
Early wetland establishment tips
Once a stormwater wetland has been constructed and
it’s a good idea to budget for revegetation or repairs
during the project’s first year.
4 to 6 inches during the beginning of the first and sec
to flood the temporary inundation zones from a source
the wetland’s banks also receive water regularly, for
to a wetland’s stability. If heavy rainfall events occur
right after a project’s completion, even the best erosion
from surface water flowing into the wetland. If this hap
matting reapplied. One way to protect against erosion
specific. If you suspect or discover wildlife damage,
Mainte
nance of Stormwater Wetlands and Wet Ponds,
Figure 18. Keep water levels relatively low during
early establishment (top), and be prepared to irri-gate
through flooding or a sprinkler system during a
drought (bottom).
Stormwater Wetland Construction Guidance 11
wetland project, it is important to be firm in sticking
ideas that can also benefit project success.
RESOURCES
588-12). Online: http://www.bae.ncsu.edu/stormwa
ter/PublicationFiles/WetlandDesignUpdate2007.pdf
and Wet Pond Maintenance (AGW-588-7). Online:
http://www.bae.ncsu.edu/stormwater/Publication
(AG-588-2). Online:
http://www.bae.ncsu.edu/storm¬water/Publication
edu/stormwater N.C. State University’s clearinghouse
for stormwater BMP guidance, including design, con
Figure 19. Adding features and topsoil in a large
wetland. Side-slopes have already been prepped, and
grass is coming up (top). After 1 year, vegetation is
successfully established (bottom).
Mature plants beautify a stormwater wetland.
established in the first growing season, plant growth
wetland a permanent home. If you find that wildlife
CONCLUSIONS
Stormwater wetlands are important stormwater BMPs
with care. Proper construction planning saves time
and money. Practices discussed and recommended in
NC STATE UNIVERSITY
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Published by
NORTH CAROLINA COOPERATIVE EXTENSION
11-CALS-2218 AGW-588-13
11/10—BS/KEL
Prepared by
Michael R. Burchell II, Assistant Professor and Extension Specialist
William F. Hunt, P.E., Associate Professor and Extension Specialist
Kristopher L. Bass, P.E., Extension Associate
Department of Biological and Agricultural Engineering
North Carolina State University
Jason Wright, P.E., Water Resources Engineer,
TetraTech, San Diego, Calif.
250 copies of this public document were printed at a cost of $595 or $2.38 per copy.